1. Field of the Invention
The present invention relates to a method of uniformly heat-treating wafers.
2. Description of the Related Art
The heat-treating apparatus is conventionally employed for use with various kinds of apparatus such as the CVD apparatus for forming thin film, the epitaxial growth apparatus and the oxide film forming apparatus or with the heat-diffusing unit of the doping apparatus in the course of manufacturing semiconductor wafers.
The heat-treating apparatus has been developed to have a heater whose inner sufficient diameter exceeds 300 mm so as to meet the trend of making diameters of semiconductor wafers larger and larger. As the diameter of the semiconductor wafer is made larger and larger, however, it becomes more difficult to control temperature in the process tube. This makes it necessary to often use the furnace of the vertical type because it is easier to control temperature on any of cross sections of the furnace.
A CVD furnace of this vertical type is disclosed in U.S. Pat. No. 4,938,691, for example. A process tube in the CVD furnace of the vertical type is provided at the bottom thereof with an opening through which a wafer boat is carried into and out of the process tube. 100-150 sheets of semiconductor wafers are mounted parallel to one another on the boat. A spiral heater made of resistant heating material such as FeCrAl alloy encloses the process tube and the wafers in the process tube are heated to a desired process temperature ranging from 700.degree. C. to 1200.degree. C. by this spiral heater. The spiral heater is enclosed by heat-insulating material so as to reduce the amount of heat radiated from the process tube (or amount of radiation heat).
In the conventional case, however the wafers on the boat are heated from outside, and the center portion of each wafer is more heated than the peripheral portion. Consequently, as is shown in FIG. 1, as the temperature in the furnace is raised from room temperature (line K) to a predetermined temperature (line P) slightly lower than a process temperature (line Q), the temperature difference between the center and peripheral portion of each wafer increases gradually. Thereafter, the temperature in the furnace is maintained at the process temperature (line Q), whereby said temperature difference decreases gradually to substantially nil.
In particular, when the wafers are heated by a heater of the type disclosed in Published Examined Japanese Patent Application No. 48-8657, which has MoSi.sub.2 heating element, they are heated so fast that the temperature difference between the center and peripheral portions of each wafer is considerably great at the predetermined temperature (line P) which is a little lower than the process temperature (line Q). When such a temperature difference occurs, the thin film formed on the wafer has but a non-uniform thickness, ultimately degrading the quality of the semiconductor devices formed on the wafer.
This non-uniformity of temperature on each wafer surface results from how the wafers are positioned and held in the process tube in the furnace of the vertical type. In order to solve the non-uniformity of temperature on each wafer surface, therefore, it is desired to improve the process of manufacturing semiconductor devices.
The larger the diameter of each wafer, the more difficult it is to space the wafer away the heater by a long distance. If the heater is a type, in particular, the wafers are heated to different extents. Consequently, the semiconductor devices made from one wafer have quality different from that the devices made from another wafer.